Heretofore single direction moving conveyors have been associated with switches that are operated to interrupt power being supplied to a conveyor drive motor upon detection of an overload or an underload condition. An example of an overload detector is disclosed in U.S. Pat. No. 3,595,372 issued July 27, 1971, to W. M. Mojden. In this patent a conveyor driving chain passes over a sprocket that is rotatably mounted on a movable support. Upon encountering a jam or overload condition in the conveyor, power imparted to the chain is effective to move the idler and its movable support to operate a switch to interrupt power being applied to a motor that drives the conveyor.
An example of a conveyor system having facilities to detect an underload condition is exemplified by U.S. Pat. No. 3,905,473 to R. D. Jones et al. issued Sept. 16, 1975. In this system a conveyor passes over an idler pulley that is mounted on a block which is slidably mounted and normally held in position through the agency of a spring. Upon development of too much slack in the conveyor, the spring acts to move the idler mounting means to actuate a switch which functions to interrupt the power applied to a conveyor drive motor.